1. Field of the Invention
The invention is in the field of electrical power production from high temperature geothermal brines which have high dissolved silica content.
2. Description of the Prior Art
Some very high temperature geothermal brine resources are known which contain large amounts of geothermal energy, but which have heretofore not been usable for the commercial production of electrical power because of a high dissolved silica content. These resources include the Salton Sea and Brawley geothermal fields in the Imperial Valley in California, where underground geothermal brine source temperatures range from about 500.degree. F. to about 620.degree. F. and the dissolved silica content is at substantially the saturated level for these brine source temperatures, ranging from about 500 to about 600 parts per million by weight. This high silica content appears to result from the nature of the porous rock formations in the high temperature brine sources in geothermal fields of this type.
Extensive flow testing of high temperature, high silica Salton Sea field brines has been carried out in a simulated geothermal power plant at Niland, Imperial County, California, where the high temperature geothermal brine is flowed up through a production well under the power of its own flashing steam, then flowed through a heat extraction facility simulating a geothermal electrical power generating plant, and then pumped back into an underground formation through a reinjection well spaced a considerable distance from the production well. This testing operation at Niland, although making use of the best state-of-the-art technology, revealed that the high, substantially saturated dissolved silica content in this type of high temperature geothermal brine resource causes so many problems in both plant equipment and reinjection equipment that it would not be practical to attempt to produce electrical power on a commercial scale from this type of geothermal resource using prior art technology.
The basis for these problems is that the saturation amount of silica lowers as the temperature of the brine drops when the brine flows up through the production well, passes through the heat extraction equipment of the plant, and is flowed down into the reinjection well. The silica saturation curve drops down only gradually from the approximately 500.degree. F. to 620.degree. F. underground source temperature as the brine flows up through the production well casing and the temperature is reduced to a typical wellhead temperature on the order of about 400.degree. F. to 500.degree. F.; and the silica saturation curve still does not drop down very steeply in relatively high temperature heat extraction equipment of the plant, for example, in a temperature range of from 400.degree. F. to 500.degree. F. at the wellhead down to perhaps 320.degree. F. to 375.degree. F. after flashing in a first stage flash vessel. However, the silica saturation curve then drops off much more steeply below these temperatures, and particularly in relatively low temperature heat extraction equipment of the plant, for example in a temperature range of perhaps 240.degree. F. to 260.degree. F. after flashing in a second stage flash vessel; and in reinjection equipment where the brine temperature may drop down as low as about 220.degree. F., which would be several degrees Fahrenheit below boiling for brines of this type which have high mineral content.
Since the silica saturation curve descends only gradually in the temperature range of the production well bore, and the silica precipitation reaction is a slow one, little or no silica scaling is likely to occur in the production well bore. This also holds true for production brine input conduits of the plant where the brine temperature is still high and residency time is brief. However, the increasingly steep silica saturation curve for the relatively large temperature drops in successive flash vessels or other heat extraction apparatus such as heat exchangers in a binary system, and the further temperature drops associated with reinjection equipment, coupled with a considerable residency time of the brine in such heat extraction and reinjection equipment, will cause silica to precipitate out of the brine in material quantities in relatively high temperature heat extraction equipment, and in large quantities in relatively low temperature heat extraction equipment and reinjection equipment. Such silica precipitation occurs principally as scaling in the form of a hard, rock-like glaze on the inner walls of flash vessels or heat exchangers in the heat extraction apparatus, in various conduits, and most importantly in the reinjection well casing where the precipitated silica causes rapid plugging that will lead to loss of the well.
Such silica scaling causes progressive reduction in the amount of geothermal brine a plant can process, and also reduction in the heat extraction efficiency for the amount of brine that is being processed. The silica scaling is so tough that it is difficult to remove, and some equipment would have to be replaced; at the same time, such attempts to remove silica scaling or replace scaled parts would involve downtime for plant equipment. Once a reinjection well becomes plugged to the extent that it will not pass a sufficient flow rate of geothermal brine back down into the aquifer for efficient plant operation, the well is lost and a new reinjection well must be prepared at great expense.
One prior art method for controlling mineral precipitation in connection with a geothermal electrical power plant was taught in U.S. Pat. No. 3,757,516 issued to Barkman C. McCabe. That patent taught the principle of deep well pumping in the geothermal brine production well and pressurization throughout the entire plant system on through reinjection to avoid loss of the heat of vaporization from that portion of the brine which would otherwise flash to steam in the production well, and incidentially to reduce mineral precipitation in the brine flow path. However, the said McCabe U.S. Pat. No. 3,757,516 was concerned only with those geothermal resources having a temperature under about 400.degree. F., where the loss of heat of vaporization would represent a loss of a considerable proportion of the available thermal energy, so that flashing in the production well bore would represent a serious energy loss in the system. Also, the mineral precipitation that was of principal concern in that patent was calcium carbonate, which, without the pumping, would be precipitated from brines having a substantial calcium oxide content, due to the release of carbon dioxide from the brine during flashing, and the chemical combining of carbon dioxide with calcium oxide to form the calcium carbonate precipitate.
The pressurization procedure taught in said McCabe U.S. Pat. No. 3,757,516, while effective to prevent some types of scaling such as from calcium carbonate, would, however, not be effective to prevent dissolved silica from precipitating out on walls of heat extraction and reinjection equipment of a geothermal power plant, as such silica precipitation depends only upon temperature reduction to put the dissolved silica in a supersaturated condition, and time for the slow silica precipitation reaction to occur. Nevertheless, the relatively low geothermal hot water or brine source temperatures to which that patent applied (below about 400.degree. F.) did not carry the large quantities of silica (even if saturated with silica) that are found at substantially the saturation level in very hot brines ranging from about 500.degree. F. to about 620.degree. F. in geothermal energy resources like the Salton Sea and Brawley geothermal fields. It is these large quantities of silica in very hot brine which heretofore have presented insurmountable problems in attempts to utilize thus huge thermal potential energy resource for the commercial generation of electrical power.
U.S. Pat. No. 4,043,129 issued to Barkman C. McCabe and Edward Zajac applied the deep well pumping concept of the earlier McCabe U.S. Pat. No. 3,757,516 to high temperature geothermal brines above about 500.degree. F. The McCabe-Zajac U.S. Pat. No. 4,043,129 taught that the advantages of deep well pumping, including avoidance of the loss of heat of vaporization and reduction of mineral precipitation, could be realized in connection with very high temperature geothermal brines by mixing a high temperature geothermal brine which might have a relatively high mineral content derived from a relatively deep well with a lower temperature brine that might have a relatively low mineral content derived from a shallower or peripheral well, to produce a brine mixture of sufficiently lowered temperature to be within the practical temperature and pressure ranges for pumping and which may also have a lowered mineral content. However, this still would not solve the serious silica scaling problem in plant and reinjection equipment where the high temperature brines had a dissolved silica content proximate saturation levels at source temperatures, the situation in the Salton Sea and Brawley geothermal fields.
Some more recent prior art work has involved the use of a reactor clarifier, a type of apparatus known in the sewage treatment art, in an attempt to reduce the silica content of high temperature, high silica geothermal brines so as to protect reinjection well equipment against silica plugging. A publication regarding such use of a reactor clarifier was made at the Second Invitational Well-Testing Symposium of October 25-57, 1978 at the University of California, Berkeley, by Robert H. Van Note, John L. Featherstone and Bernard Pawlowski, entitled "A Cost-Effective Treatment System for the Stabilization of Spent Geothermal Brines". However, this only involved the lower temperature tail end part of a proposed geothermal electrical power plant, and did not have any effect on the silica precipitation problem in heat extraction apparatus such as steam separators or heat exchangers. Thus, where a plurality of steam separators in successively lowering temperature and pressure ranges were contemplated for a geothermal electrical power plant, the use of a reactor clarifier in a downstream lower temperature position could not check the silica from starting to precipitate out onto walls of a first stage steam separator or from precipitating out in large quantities on walls of a second stage steam separator, as well as in associated conduits.